1. Field of the Invention
The present invention relates to an OLED (organic light emitting device) panel obtained by forming an OLED on a substrate and sealing the OLED between the substrate and a cover member. The invention also relates to an OLED module in which an IC including a controller, or the like, is mounted to the OLED panel. In this specification, xe2x80x98light emitting devicexe2x80x99 is the generic term for the OLED panel and for the OLED module. Electronic equipment using the light emitting device is also included in the present invention.
2. Description of the Related Art
An OLED emits light by itself, and thus, has high visibility. The OLED does not need a backlight necessary for a liquid crystal display device (LCD), which is suitable for a reduction of a light emitting device in thickness. Also, the OLED has no limitation on a viewing angle. Therefore, the light emitting device using the OLED has recently been attracting attention as a display device that substitutes for a CRT or the LCD.
The OLED includes a layer containing an organic compound in which luminescence generated by application of an electric field (electroluminescence) is obtained (organic light emitting material) (hereinafter, referred to as organic light emitting layer), an anode layer and a cathode layer. A light emission in returning to a base state from a singlet excitation state (fluorescence) and a light emission in returning to a base state from a triplet excitation state (phosphorescence) exist as the luminescence in the organic compound. The light emitting device of the present invention may use one or both of the above-described light emissions.
Note that, in this specification, all the layers provided between an anode and a cathode of the OLED are defined as the organic light emitting layers. The organic light emitting layers specifically include a light emitting layer, a hole injecting layer. an electron injecting layer, a hole transporting layer, an electron transporting layer and the like. The OLED basically has a structure in which an anode/a light emitting layer/a cathode are laminated in order. Besides this structure, the OLED may take a structure in which an anode/a hole injecting layer/a light emitting layer/a cathode are laminated in order or a structure in which an anode/a hole injecting layer/a light emitting layer/an electron transporting layer/a cathode are laminated in order.
The problem in putting a light emitting device into practice is lowering in luminance of OLED which accompanies degradation of an organic light emitting material.
Organic light emitting materials are weak against moisture, oxygen, light, and heat, which accelerate degradation of the organic light emitting materials. The rate of degradation of an organic light emitting material depends specifically on the structure of a device for driving the light emitting device, characteristics of the organic light emitting material, materials of electrodes, conditions in a manufacture process, how the light emitting device is driven, etc.
Even when the voltage applied to the organic light emitting layer is constant, the luminance of the OLED is lowered as the organic light emitting layer degrades, and an image to be displayed is therefore become unclearly. In this specification, a voltage applied to an organic light emitting layer from a pair of electrodes is called an OLED drive voltage (Vel).
When an image is displayed in color by using three types of OLEDs that emit red (R) light, green (G) light, and blue (B) light respectively, different organic materials are used to form organic light emitting layers of OLEDs of a plurality of colors. Accordingly the rate of degradation of organic light emitting layer may vary between OLEDs of a plurality of colors. Then difference in luminance between OLEDs of a plurality of colors will be apparent as time passes, making it impossible for the light emitting device to display an image in desired colors.
The present invention has been made in view of solving the above, and an object of the present invention is therefore to provide a light emitting device capable of displaying a clear image in desired colors by controlling lowering of luminance of OLED when its organic light emitting layer is degraded.
The present inventors have taken notice of the fact that the luminance of OLED is lowered by degradation less when light is emitted with a current flow to an OLED kept constant than when light is emitted with the OLED drive voltage kept constant. In this specification, a current flowing into an OLED is called an OLED drive current (Iel).
FIG. 2 shows changes in luminance of OLED when the OLED drive voltage is constant and when the OLED drive current is constant. As shown in FIG. 2, the luminance is lowered by degradation less when the OLED drive current is constant.
Therefore, the present inventors have devised a light emitting device in which the OLED drive current is kept constant through correction of OLED drive voltage when the OLED drive current is lowered by degradation.
Specifically the light emitting device of the present invention has first means of measuring the OLED drive current, second means of calculating from a video signal an ideal OLED drive current value (reference value), third means of comparing the measured value with the reference value, and fourth means of correcting the OLED drive voltage to reduce the difference between the measured value and the reference value.
With the above structure, the present invention can keep the OLED drive current constant when the organic light emitting layer is degraded to prevent the luminance from lowering. As a result, the light emitting device of the present invention can display a clear image.
If the light emitting device is to display an image in color using three types of OLEDs that emit red (R) light, green (G) light, and blue (B) light, respectively, the OLED drive current may be measured for OLEDs of a plurality of colors separately to correct their respective OLED drive voltages. This structure makes it possible to keep the luminance of light of a plurality of colors balanced and display in desired colors when the rate of degradation of organic light emitting layer varies between OLEDs of a plurality of colors.
The temperature of organic compound layer is influenced by the outside temperature and heat generated by the OLED panel itself. Generally, the amount of current flowing in an OLED varies depending on the temperature. FIG. 3 shows a change in voltage-current characteristic of an OLED when the temperature of its organic light emitting layer is changed. With the voltage kept constant, the OLED drive current is increased as the temperature of the organic light emitting layer goes high. Since the OLED drive current is in proportion to the OLED luminance, the luminance of the OLED becomes higher as the OLED drive current becomes larger. In FIG. 2, when the voltage is constant, the luminance goes up and down in an about 24-hour cycle reflecting the temperature difference between day and night. However. the light emitting device of the present invention can keep the OLED drive current constant when there is a change in temperature of the organic light emitting layer by correcting the OLED drive voltage. Therefore the luminance can be kept constantly irrespective of temperature change and an increase in power consumption accompanying temperature rise can be prevented.
Generally, temperature change brings varying degrees of changes in OLED drive current to different types of organic light emitting materials and, therefore, in color display, the luminance could be changed by temperature change differently for OLEDs of a plurality of colors. However, the light emitting device of the present invention can keep the luminance constant irrespective of temperature change to thereby keep the luminance of light of a plurality of colors balanced. An image can thus be displayed in desired colors.
The light emitting device of the present invention is convenient because the OLED current can be measured without disturbing the display on the screen the viewer is viewing.
In a common light emitting device, the electric potential of a wiring line used to supply a current to pixels may be lowered as the wiring line becomes longer because of the resistance of the wiring line itself. This electric potential is lowered to widely varying degrees depending on an image to be displayed. When the ratio of higher gray scale pixels to all of the pixels that receive a current from the same wiring line is large, in particular, the current flowing through the wiring lines is increased in amount to make lower of electric potential noticeable. When the electric potential is lowered, a smaller voltage is applied to the OLED of each pixel to reduce the amount of current supplied to each pixel. Therefore, the amount of current supplied to one pixel is changed as well as the gray scale number thereof when the gray scale number of the other pixels that receive a current from the same wiring line as the one pixel is changed, making it impossible for the one pixel to keep a constant gray scale. In the light emitting device of the present invention, on the other hand, the measured value and the reference value are obtained to correct the OLED current each time a new image is displayed. Therefore, a desired gray scale number is obtained for every new image through correction.